1. Field of the Invention
The present invention relates to a liquid crystal (LC) shutter.
2. Description of the Related Art
In recent years, an LC shutter has come into use in an electro-photographic printer or the like, as a light-controlling element for transmitting and shielding light.
The LC shutter comprises a first substrate, a second substrate located parallel to the first substrate, a layer of LC material sealed in the gap between the first and second substrates, and a pair of polarizing plates formed on the outer surfaces of the two substrates, respectively. The shutter further comprises a number of small signal electrodes aligned in one direction on the inner surface of the first substrate, a common electrode formed on the inner surface of the second substrate, and aligning films formed on the signal electrodes and the common electrodes. Those portions of the layer of LC material, which are located between the common electrode and the signal electrodes, function as microshutters for transmitting and shielding light.
Various types of LC shutters are known. U.S. Pat. No. 4,386,836 discloses a TN (Twisted Nematic) LC shutter. U.S. Pat. No. 4,745,433 discloses a G.H. (Guest-Host) LC shutter. U.S. Pats. No. 4,569,574 and No. 4,595,259, and Japanese Patent Disclosure No. 58-176620 disclose a birefringent LC shutter. Further, U.S. Pat. No. 4,591,886 discloses a ferroelectric LC shutter. Of these types, the birefringent LC shutter attracts much attention for two reasons. First, it can transmit a great amount of ON light, thus displaying a high-contrast image. Secondly, it can respond to drive signals at high speed. The ferroelectric LC shutter attracts much more attention, because it can respond to drive signals, theoretically, at much higher speed.
For the above reason, studies are being made on the possibility of using these types of LC shutters as the light-controlling element in electro-photographic printers.
In the birefringent LC shutter or the ferroelectric LC shutter, the LC layer is sealed between the substrates, and molecules of the LC layer are orientated in a desired direction uniformly. The polarizing plates are arranged such that their polarizing axes (either the light-transmitting axes or the light-absorbing axes) extend parallel or at right angles. At least one of the polarizing axes intersects with a direction in which the molecules of LC material are orientated. When an electric field is applied to the LC material, the birefringence of the LC layer changes, whereby the light passing through the output polarizing plate is controlled. In short, either the birefringent LC shutter or the ferroelectric LC shutter is a birefringent effect type LC element which can transmit and shield light by virtue of its birefringence effect.
The LC shutter used as light-controlling element in an electro-photographic printer comprises two rectangular substrates located parallel to each other, and liquid crystal material sandwiched between the substrates and a number of microshutter elements are arranged in a line or more, in a predetermined direction. The opposing inner surfaces of the substrates are subjected to an aligning treatment such that the molecules of liquid crystal material are orientated in the predetermined direction. More specifically, these surfaces of the substrates are covered by aligning films, these aligning films are rubbed in the lengthwise direction of the substrates. Two polarizing plates are formed on the outer surfaces of the substrate, respectively. These polarizing plates are positioned such that the polarizing axis of at least one of the plates intersects, at a desired angle, with the direction of the LC layer.
FIG. 1 is a plan view showing a conventional LC shutter utilizing the birefringence effect. This figure shows directions of aligning treatment of the substrates, and also the light-transmitting axes of the polarizing plates of this LC shutter. In FIG. 1, the directions of the aligning treatments of the upper and lower substrates 1 and 2 are shown by arrows A1 and A2. Light-transmitting axes of the polarizing plates B1 and B2 are shown by arrows, respectively. The directions Al and A2 are parallel to the axis of the LC shutter, and the axes B1 and B2 intersect with each other at right angles and with the directions A1 and A2 at 45.degree., respectively. Alternatively, the directions A1 and A2 can be at right angles to the axis of the shutter, and the light-transmitting axes B1 and B2 can be parallel to each other.
The LC shutter illustrated in FIG. 1 is irradiated with light from a light source, and controls the light emitted from a light source, transmitting or shielding the light. The shutter is heated by the light to a temperature higher than the ambient temperature. Naturally, the LC material is heated. The operating characteristic of the LC shutter depends on the temperature of the LC material. To stabilize the operation characteristic, it is necessary to maintain the LC material at an optimal temperature. To this end, heaters are located along the array of shutter elements, on both sides of the array. When the temperature of the shutter elements is too low, the heaters are turned on to heat the elements to the optimal value.
When the temperature of the material rises, the amount of light the shutter transmits in the ON state (hereinafter called "ON-amount of light"), and the amount of light the shutter transmits in the OFF state (hereinafter called "OFF-amount of light") change, despite the use of the heaters. Hence, the contrast of the image formed by the LC shutter decreases.
This problem inherent in the conventional LC shutter will be discussed in greater detail, with reference to FIGS. 2 and 3. FIGS. 2 and 3 are graphs representing the relationship which the ON-amount of light and the OFF-amount of light, on the one hand, and the temperature of the center portion of the LC shutter shown in FIG. 1, on the other, have when heaters are used to maintain the LC shutter at 50.degree. C., while the ambient temperature is 20.degree. C. FIG. 2 is a graph illustrating the operating characteristic of a conventional LC shutter, wherein the directions A1 and A2 are parallel to the direction in the shutter elements are arranged, and the polarization axes B1 and B2 intersect at 45.degree. with this direction. Either LC shutter of FIGS. 2 and 3 are driven in the 1/2 duty, double-frequency addressing scheme. The shutter elements of the LC shutter remain in the ON state (i.e., the light-transmission state) as long as an ON electric field and an OFF electric field are alternately applied to them. The shutter elements goes into an OFF state (i.e., the light-shielding state) when the OFF electric field is continuously applied to them. As can be understood from FIGS. 2 and 3, the operating characteristic of either LC shutter becomes stable when the center portion is heated by the heaters to 50.degree. C. upon lapse of a specific time. In the case of the shutter wherein the directions A1 and A2 are parallel to the direction in which the shutter elements are aligned, and the polarizing axes B1 and B2 intersect at 45.degree. to this direction, the ON-amount of light and the OFF-amount of light increase with time and become stable upon lapse of about 20 minutes after the temperature of the shutter has risen to 50.degree. C., as is evident from FIG. 2. In the case of the shutter wherein the directions A1 and A2 intersect at right angles with the direction in which the shutter elements are aligned, and the polarizing axes B1 and B2 intersect at 45.degree. to this direction, the ON-amount of light and the OFF-amount of light decrease with time and become stable upon lapse of about 20 minutes after the temperature of the shutter has risen to 50.degree. C., as is evident from FIG. 3.
As is evident from FIGS. 2 and 3, in either type of the LC shutters, the OFF-amount of light changes with time and the ON-amount of light changes along with the OFF-amount of light since the shutter elements remain in the ON state as long as an ON electric field and an OFF electric field are alternately applied to them. Further, in either LC shutter, the OFF-amount of light varies with the ambient temperature.
As has been described above, the conventional LC shutter which make use of the birefringence effect of LC material is disadvantageous in that the OFF-amount of light changes with the ambient temperature despite that the center portion of the shutter is maintained at a predetermined temperature.